Electromagnetic components are electrical components that make use of the magnetic fields associated with electrical current passing through such components. Examples of electromagnetic components include inductors, transformers, and motor windings. Electromagnetic components are used in a variety of electrical devices, and it is often necessary to estimate the electrical current flowing through such components. For example, the current through a transformer winding of a switched-mode power supply (SMPS) must often be estimated, the current through a winding within an electric motor may need to be estimated, or it may be necessary to estimate the current through an inductor in some other type of electrical circuit.
In an SMPS configured in an isolated topology, the current through transformer windings must often be estimated to ensure that the transformer core does not reach a saturation level of magnetic flux, and for purposes of controlling the output power provided to a load. In a resonant or semi-resonant SMPS, it is desirable that power switches only be disabled when there is no (or almost no) current flowing through them. This requires that the current flowing through a power switch be estimated. Because this current also flows through the winding(s) of a transformer (or center-tapped inductor), this current may be estimated based upon the current through such winding(s).
Many techniques for estimating current flowing through electromagnetic components are available, including techniques that make use of shunt resistors, direct current resistance (DCR) circuits, Hall effect sensors, current transformers, and power switches. As summarized below, such conventional current sensing techniques have drawbacks in many applications.
Perhaps the most straightforward technique for measuring current through a component, electromagnetic or otherwise, is to place a shunt resistor in series with the component. The voltage across the shunt resistor is measured and the current is calculated using Ohm's law. While such techniques are simple, the shunt resistor wastes power due to the I2R losses through it, and the shunt resistor undesirably increases the effective resistance of the component. In order to minimize these effects, a shunt resistor having a very low resistance is often used, and an amplifier is used to amplify the voltage across the shunt resistor so that the voltage is in a usable range for, e.g., an analog-to-digital converter (ADC). This can result in a noisy measured voltage and a non-trivial temperature dependence for the resistance, both of which lead to inaccurate results.
A direct current resistance (DCR) sense circuit addresses some of the deficiencies associated with use of a shunt resistor. A DCR circuit makes use of the inherent resistance of an inductor (or, similarly, a winding of a transformer), together with a matching circuit, including, typically, a capacitor and a sense resistor. The capacitor(s) and sense resistor(s) are chosen such that a voltage output from the DCR circuit may be used to estimate the current through an inductor (or transformer winding). While such techniques avoid the power loss associated with a shunt resistor, these techniques present a variety of accuracy issues. The capacitor(s) within the DCR sense circuit typically have large tolerance values, meaning that the circuit must often be painstakingly tuned. Because the capacitance and inductance within the DCR sense circuit may have significant (and different) temperature dependencies, the current estimates often exhibit significant temperature-dependent errors. Furthermore, a DCR sense circuit produces results that are highly frequency-dependent, due to the matching capacitor(s) and the inductance of the electromagnetic component (e.g., inductor or transformer winding).
A Hall effect sensor measures the strength of the magnetic field induced by current flow through a nearby conductor in order to determine the current passing through that conductor. Hall effect sensors are typically bandwidth-limited (e.g., they can only be used to measure signals <150 kHz) and, hence, are not appropriate for applications using relatively high frequency signals (e.g., many SMPS). The current estimates produced by Hall effect sensors exhibit inaccuracies due to a variety of reasons. For example, physical inaccuracies and material non-uniformities in a sensor may lead to an offset in the output voltage used for indicating the sensed current. Additionally, the estimates provided by a Hall effect sensor drift as temperature changes, and are subject to inaccuracies caused by external magnetic fields that interfere with the field from the intended electromagnetic component (e.g., inductor). Furthermore, Hall effect sensors are expensive and consume potentially valuable area on a printed circuit board (PCB), or similar, within an electrical device.
Yet another technique for measuring current is to add a current transformer in series with the electromagnetic component. Such a transformer couples a magnetic field that is proportional to a primary current into a secondary winding. A burden resistor is coupled across the secondary winding so that the secondary current may be converted into a voltage that may be measured and translated into a current estimate. The secondary winding, and its associated burden resistor, typically have a current that is much lower than that flowing through the primary winding (and the electromagnetic component), and a voltage that is much higher than (the undesired) voltage generated across the primary winding. Use of a current transformer has the disadvantages that it requires an alternating current (AC), it produces AC insertion loss, it has a fairly large size, and it is expensive.
For electrical devices that include electromagnetic components through which current also flows through a power switch such as a metal-oxide semiconductor field-effect transistor (MOSFET), the MOSFET current may be estimated. One technique for doing so makes use of a drain-to-source voltage measurement and a characteristic drain-source resistance (Rdson) when the MOSFET is enabled and conducting. While such techniques do not incur additional unnecessary power loss, as do many of the above techniques, current estimates based upon of Rdson exhibit significant noise, have a high temperature dependency, and produce larger errors due to production variations for the MOSFET(s). Because the MOSFET must be enabled to obtain a meaningful drain-to-source voltage, such techniques are only able to estimate current when the MOSFET is conducting.
Yet another technique uses a sensing transistor (current mirror) to sense the current through a power switch (e.g., MOSFET). Such techniques make use of a sensing transistor (e.g., MOSFET) that is configured in parallel with a power switch, and through which a current proportional to that in the power switch flows. Such techniques exhibit high noise, require a high design effort, and current measurements are only possible during the “on” time of the power switch.
Accordingly, there is a need for improved techniques for estimating current through an electromagnetic component, and which mitigate at least some of the problems described above for conventional current estimation techniques. Such improved techniques should not incur any significant power loss, should require minimal extra circuitry, and should provide accurate current estimates.